A mobile communication system is one in which mobile or portable user terminals, such as mobile telephones, portable radios or radios on vehicles, herein collectively referred to as ‘mobile stations’, or ‘MSs’ (‘MS’ in the singular), can communicate via a network infrastructure which generally includes fixed installations including at least one fixed base station, known in the art as a ‘base transceiver station’ or ‘BTS’, and usually various sub-systems for management and control of the system including the at least one BTS. The system may for example be a cellular one including a plurality of BTSs wherein each BTS serves MSs in a given region or area known as a ‘cell’ or ‘site’ by radio communication. The cells of neighbouring BTSs in such a system are often overlapping.
Signals sent from MSs to their serving BTS in a mobile communication system are known as ‘uplink’ signals. Signals sent from a BTS to MSs are known as ‘downlink’ signals. Uplink and downlink signals may be, and usually are, sent on different channels, e.g. with different carrier frequencies.
A mobile communication system, especially a cellular system, may be a trunked system in which radio channels of the system are shared between MSs for different communications, and each channel is assigned for a particular communication for a temporary period only.
Communications of different types in a mobile communication system, originating either at a BTS or at a MS, may be sent on different channels dedicated to the different communication types. For example, the channels employed may include control channels and traffic channels in which communications transmitted comprise respectively control signals and traffic signals. There may be different channels for different traffic types for example communications of voice, short data and packet data information. In a system operating according to a TDMA (Time Division Multiple Access) protocol, the different channels may be provided by different specified time slots or frames of slots of a single RF carrier, the different time slots or frames of slots being provided within a timing structure used by all terminals of the system.
Users of MSs generally require a high grade of service and reliable connectivity whilst using the system for various services such as telephony calls, dispatch calls and data transfers, especially more advanced data transfers such as transmission of picture or video images.
Generally, in a cellular system, it is desirable therefore for the MSs to be served by a selected BTS which can provide good radio signals to and from the MS. Since MSs can move from one region to another it is known for the MSs to monitor signals from different BTSs, in order to operate a procedure to determine whether it would be worthwhile, in order to receive better service, to switch from a current serving BTS to another one and if appropriate to carry out such a switch. In the art, the procedure to monitor for and to make determinations regarding such a possible switch is known as a ‘cell re-selection’ procedure. This usually operates in two stages, namely (i) a first stage in which the mobile station draws up a list of candidate non-serving BTSs to which it could potentially switch to as its serving BTS, including a preferred non-serving BTS which is selected to be top of the list; and (ii) a second stage in which measurements are carried out comparing signals from the preferred non-serving BTS with those from the current serving BTS and determining whether certain criteria are met which require a switch to the preferred BTS to be made. Where such criteria are met, a procedure known as ‘handover’ or ‘handoff’ is carried out to effect such a switch so that the MS becomes served by the preferred BTS.
Any wireless link is a potential target for interference generated either accidentally or intentionally (e.g. by a hostile activity). The risk of accidental interference may be increased by the coexistence of different systems or devices in the same area sharing the same (or a close) frequency. Moreover, in a cellular communication system, accidental interference can be caused by an inappropriate frequency re-use strategy or by unusual propagation conditions. Accidental interference can however be eliminated by careful planning and design of the system.
Interference by jamming causes a greater problem, since when this occurs all communications on the channel or channels being jammed may become unintelligible noise rather than useful signals. The channels providing the more sensitive communication links, i.e. those which are more attractive to for a would be hostile party to jam, are the downlink channels, particularly the downlink control channel. Every service to MSs provided by the infrastructure is established using information exchange on the downlink channels from the serving BTS. In the event that a downlink control channel is blocked by interference due to jamming, no service can be provided. The risk of jamming of the downlink channels is increased because the transmission by a BTS on its downlink channels, usually on a given carrier frequency, is substantially continuous and therefore can be easily detected and analysed by receiving devices being used in a hostile jamming activity. In contrast, on the uplink channel, there is in general no continuous transmission; transmission is performed by MSs only when a service is being been used or being requested for use by an MS.
EP-A-1304895 describes a known method of detecting interference by jamming to a signal sent by a downlink channel. A polling signal is sent by a BTS to MSs. The MSs automatically send response messages to the BTS if they receive the polling signal. A condition of interference by jamming is deemed to exist if no response messages are received by the BTS, indicating that the MSs did not satisfactorily receive the polling signal.